This invention relates to a magnetron drive power supply with a magnetron of a microwave oven, etc., as a load.
Magnetron drive power supplies in related arts will be discussed with reference to the accompanying drawings. FIG. 29 is a circuit diagram of a magnetron drive power supply in a related art. The magnetron drive power supply in the related art once converts a commercial power supply 1 of AC into DC voltage through a diode bridge 2, an inverter circuit 5 generates a high-frequency voltage in a primary winding of a high-voltage transformer 6 by turning on and off semiconductor switch elements 3 and 4, and the high-voltage transformer 6 excites a high-frequency high voltage in a secondary winding. This high-frequency high voltage is rectified to DC high voltage by a high-voltage rectification circuit 7 and the DC high voltage is applied to a magnetron 8. The magnetron 8 is driven at the DC high voltage and generates a radio wave of 2.45 GHz.
FIG. 30 is a drawing to show the operation waveform of the magnetron drive power supply in the related art. AC voltage V1 of the commercial power supply 1 is rectified to a DC voltage through the diode bridge 2. An inductor 9 and a capacitor 10 make up a smoothing circuit; the capacitor 10 has a capacity to such an extent that it can hold DC voltage with respect to the inverter circuit 5 operating in the range of 20 kHz to 50 kHz to miniaturize the inverter circuit 5, and does not have a capability of smoothing for the frequency of the commercial power supply 1 (50 Hz or 60 Hz). Thus, voltage V10 of the capacitor 10 shows a waveform provided by simply full-wave rectifying the commercial power supply 1 and shows a pulsation waveform fluctuating from almost 0 voltage to the maximum voltage of the commercial power supply 1. Since the inverter circuit 5 operates based on the pulsating voltage V10 of the capacitor 10, the envelope waveform of the high-frequency voltage generated in the primary winding of the high-voltage transformer 6 becomes a waveform as shown in V6 (Lp) and in the time period over which the voltage V10 of the capacitor 10 is low, likewise only a low voltage can be generated.
On the other hand, the operation characteristic of the magnetron 8 shows a nonlinear voltage current characteristic such that an anode current does not flow if a predetermined voltage or more is not applied between an anode and a cathode, as shown in FIG. 31. Therefore, in the time period over which the voltage generated in the primary winding of the high-voltage transformer 6 is low, the voltage excited in the secondary winding also becomes low at the same time and thus in the waveform of a voltage V8 applied to the magnetron 8, a time period over which the voltage does not reach VAK (TH) occurs, as shown in the figure. In the time period, the magnetron 8 stops oscillation and thus power is not consumed in the magnetron 8 of the load and thus a current I1 of the commercial power supply 1 does not flow. Consequently, the waveform of the current I1 of the commercial power supply 1 becomes a waveform having much distortion having time periods over which the current becomes 0, as shown in FIG. 30, causing the power factor of the magnetron drive power supply to be lowered and a harmonic current to be generated in input current.
To solve such a problem, a circuit configuration shown in FIG. 32 is proposed wherein an active filter circuit 13 is placed preceding an inverter circuit 5 for improving the power factor of the input current and suppressing the harmonic. The active filter circuit 13 forms a so-called step-up chopper circuit and can control step-up voltage based on the on time ratio of a semiconductor switch element 17.
The operation will be discussed with reference to FIG. 33. The voltage of a commercial power supply 1 shows an AC voltage waveform as shown in V1. The active filter circuit 13 controls voltage provided by full-wave rectifying the AC voltage V1 through a diode bridge 2 by turning on/off the semiconductor switch element 17, thereby generating step-up voltage in a capacitor 15. The step-up voltage V15 changes in ripple factor depending on the capacity of the capacitor 15, but can be prevented from lowering completely to 0 like V10 in the configuration in FIG. 29. Thus, voltage V6 (Lp) generated in the primary winding of a high-voltage transformer 6 can be generated a predetermined value or more if the voltage of the commercial power supply 1 is in the proximity of 0. Consequently, it is made possible to always hold the voltage applied to the magnetron 8 at oscillation-possible voltage or more. Consequently, an input current 11 can be made a waveform roughly like a sine wave having no time periods over which the current becomes 0, as shown in the figure, and it is made possible to improve the power factor of the input and suppress a harmonic current.
However, in such a configuration, the active filter circuit 13 is added to the inverter circuit 5 and the power conversion process becomes rectification to boosting to harmonic generation (inverter circuit) to high-voltage rectification. Thus, the power conversion process grows and degradation of the conversion efficiency and upsizing of the circuitry introduce a problem.
Then, JP-A-10-271846 discloses a configuration intended for sharing components and circuit functions. FIG. 34 is a circuit diagram to show the circuit configuration in JP-A-10-271846. According to the circuit configuration, the boosting function operation and the inverter function operation are performed at a time for improving the power factor of input and simplifying the circuit configuration. FIGS. 35 and 36 are drawings to describe the circuit operation. FIGS. 35(a) to (d) are drawings to describe energization paths as semiconductor switch elements Q1 and Q2 are turned on and off, and FIG. 36 is an operation waveform chart corresponding thereto. The circuit operation will be discussed with reference to FIGS. 35 and 36. For convenience of the description that follows, the voltage polarity of a commercial power supply 1 is in the direction shown in the figure and the semiconductor switch element Q2 is on in the beginning. When the semiconductor switch element Q2 is on, a current flows over a path of a capacitor C2 to the commercial power supply 1 to an inductive load circuit 19 to the semiconductor switch element Q2 as shown in FIG. 35 (a), and a current IQ2 of the semiconductor switch element Q2 increases monotonously as shown in FIG. 36(a). If the semiconductor switch element Q2 is turned off in a predetermined time, the current path makes a transition to the state in FIG. 35(b) and a capacitor C1 is charged as a current flows over a path of a diode D2 to the commercial power supply 1 to the inductive load circuit 19 to a diode D3 to the capacitor C1. When all energy stored in the inductive load circuit 19 is emitted, a current flow over a path of the capacitor C1 to the semiconductor switch element Q1 to the inductive load circuit 19 to the commercial power supply 1 to the capacitor C2 in FIG. 35(c) with the capacitor C1 as a power supply. If the semiconductor switch element Q1 is turned off in a predetermined time, the inductive load circuit 19 attempts to allow a current to flow in the same direction and thus a current flow over a path shown in FIG. 35(d) (commercial power supply 1 to capacitor C2 to diode D4 to inductive load circuit 19) and the capacitor C1 is charged by energy stored in the inductive load circuit 19. When all energy stored in the inductive load circuit 19 is emitted, again a current flows over the path in FIG. 35(a) and the circuit operation is continued. Although not disclosed in JP-A-10-271846, the capacity relationship as shown in Expression 1 is required between the capacitors C1 and C2 to realize the operation:
C1 greater than  greater than C2xe2x80x83xe2x80x83(Expression 1) 
To satisfy the relation, a capacitor capable of covering a large capacity such as an electrolytic capacitor needs to be used as the capacitor C1
Such operation is performed, whereby the current from the commercial power supply 1 can be allowed to flow over roughly all regions of the power supply period for improving the power factor of input current, suppressing harmonics, and simplifying the circuitry.
Inductor 9 and capacitor 10 make up a smoothing circuit; the capacitor 10 has a capacity to such an extent that it can hold DC voltage with respect to the operation frequencies (20 kHz to 50 kHz) under the present circumstances where miniaturization of the inverter circuit 5 is advanced, and does not have a capability of smoothing for the frequency of the commercial power supply 1. Thus, as shown in FIG. 30, the voltage V10 of the capacitor 10 shows a waveform provided by simply full-wave rectifying the commercial power supply 1 and shows a pulsation waveform fluctuating from almost 0 voltage to the maximum voltage of the commercial power supply 1. Since the inverter circuit 5 operates based on the pulsating voltage V10 of the capacitor 10, the envelope waveform of the high-frequency voltage generated in the primary winding of the high-voltage transformer 6 becomes a waveform as shown in V6 (Lp) and in the time period over which the voltage V10 of the capacitor 10 is low, likewise only a low voltage can be generated. That is, a time period occurs over which the voltage does not reach threshold value VAK (TH) oscillating in the magnetron 8 having a nonlinear characteristic. In the time period, the magnetron 8 stops oscillation and thus power is not consumed in the magnetron 8 of the load and thus the current I1 of the commercial power supply 1 does not flow and becomes a waveform having much distortion having time periods over which the current becomes 0, resulting in lowering of the power factor and generating of a harmonic current in the input current.
Thus, a large number of methods are proposed wherein a step-up chopper circuit is used as a circuit configuration to complement voltage in the vicinity of the valley of a pulsation waveform from commercial power supply, components and circuit configuration are shared from the viewpoint of reducing the number of parts and miniaturizing, and the boosting function operation and the inverter function operation are performed as a time; JP-A-10-271846 discloses a representative one. FIG. 34 is a circuit diagram to show the circuit configuration in JP-A-10-271846. However, a load circuit 19 in JP-A-10-271846 is a component consuming small power like an electric discharge lamp and in a power supply unit for handling large power as in a microwave oven, a drive signal for turning on/off semiconductor switch elements Q1 and Q2 for governing the boosting operation and the inverter operation does not require a time period for charging and discharging a capacitor for boosting, so-called dead time. Further, adjustment of heating power (power consumption) such as strong, medium, weak in heating setting as in a microwave oven is not required and thus particular attention need not be given to control of the drive signal of the semiconductor switch element Q1, Q2 in the 0 voltage part and the maximum voltage part of the commercial power supply 1 or the instant at which the polarity of the commercial power supply 1 changes.
However, the above-described configuration in the related art involves the following problems and cannot sufficient provide high circuit efficiency:
In the operation waveform chart of FIG. 36, the current flowing through the diode D2 is a current shown in ID2. The voltage applied to the diode D2 changes as VD2. The current of the diode D2 becomes ideally 0 at the timing at which a transition is made from the time period in FIG. 36(b) to that in (c), but an actual diode produces a recovery current at the turn off time. When the recovery current occurs, a switching loss is produced in the diode as the product with the applied voltage. Therefore, a characteristic of high switching speed Trr is required for the diode D1, D2. However, the forward on voltage VF, another diode characteristic, of a diode having the characteristic of high switching speed Trr tends to become high, in which case the on loss at the energization time becomes large. Consequently, the loss of the diode D1, D2 becomes large and the total efficiency of the circuit cannot be made sufficiently high.
However, the configuration shown in the related art example disclosed in JP-A-10-271846 is intended for a lighting unit and the conversion power of the lighting unit is about 100W to 200W at the maximum. Therefore, as the current flowing through the circuit, only a minute current of about several A flows and thus if the diode is designed so that the forward on voltage VF becomes high as design of attaching importance to the switching speed, it is possible to design without much increasing the loss of the diode.
On the other hand, the magnetron drive power supply used with a microwave oven, etc., handles large power of about 1000 W to 1500 W as conversion power and thus a large current of 40 A to 50 A flows at the maximum as the current flowing through circuitry. Thus, if a diode is designed with importance attached to the switching speed, the forward on voltage VF becomes high and thus the loss when the diode conducts (conduction loss) becomes large, reducing the effect of decreasing the loss by increasing the switching speed. Since the cooling capability of a home microwave oven is limited naturally because of the factors of the size and costs of the microwave oven, it becomes necessary to upsize the diode or use a large-sized radiation fan to radiate heat under a limited cooling condition in order to increase the switching speed and suppress a rise in the forward on voltage VF. Thus, in the magnetron drive power supply, raising the conversion efficiency and a decrease in the loss occurring in each part of circuitry become indispensable conditions. Therefore, applying the configuration shown in the related art example to the magnetron drive power supply involves extreme difficulty from the viewpoint of decreasing the loss. Thus, to apply the configuration to the magnetron drive power supply, it becomes necessary to configure such circuitry of suppressing an increase in the switching loss of the diode and that in the on loss. Because of the magnitude of the conversion power, if an electrolytic capacitor is used with the magnetron drive power supply, an electrolytic capacitor of a high capacity and high dielectric strength is required to suppress the pulsation current of the electrolytic capacitor. This results in upsizing the power supply itself, thus inducing upsizing the microwave oven installing the magnetron drive power supply, and the effect of reducing the size and weight of the magnetron drive power supply by the high-frequency switching operation is impaired.
It is therefore a first object of the invention to provide a magnetron drive power supply to make it possible to suppress the distortion of input current, suppress occurrence of harmonics, raise the power factor of input, simplify circuitry, and improving the circuit efficiency if large power of 1 kW or more is converted.
The configuration as described above involves the following problems: In control of a machine actually handling large power, such as a microwave oven, if a circuit configuration is used in which the on/off timing of a semiconductor switch element needs to be switched according to the polarity of power supply voltage, it becomes extremely important to control a drive signal at the polarity change point where the polarity changes, because if charging and discharging of a charge-up capacitor are not well switched based on the duty ratio or the switch timing when one semiconductor switch element for governing a step-up charge-up function and an inverter function and another semiconductor switch element for governing only the inverter function are switched at the polarity change point, needle-like distortion occurs in the vicinity of the polarity change point in input current. Formerly, in such a circuit configuration, a load circuit, such as an electric discharge lamp, consumes small power and has a minute current value and the capacity of the charge-up capacitor is also small and thus input current distortion was scarcely observed. However, with a load circuit consuming large power, such as a microwave oven, it is feared that the input current waveform will become largely distorted, that the power factor will be lowered, and that the harmonic component will grow.
Further, the magnetron drive power supply making it possible to suppress the distortion of input current, suppress occurrence of harmonics, and raise the power factor of input requires two flywheel diode containing semiconductor switches and two rectification diodes. If housing the rectification diodes in one package is adopted as an inexpensive configuration, generally the element of such a configuration is less frequently used and thus cost reduction is not expected. Then, it is possible to use a method of applying a general-purpose rectification bridge diode as shown in FIG. 53; although the method can be made more inexpensive than the above-mentioned method, the number of elements is increased and the method is not considered to be the best.
It is therefore a second object of the invention to provide an inexpensive magnetron drive power supply which has a simple configuration and is excellent in cooling capability.
To solve the above-described problems, according to the invention, there is provided a magnetron drive power supply wherein a series connection body of first and second semiconductor switch elements that can be brought into reverse conduction and a series connection body of first and second diodes are connected in parallel, first and second capacitors are connected in parallel to the first and second diodes, a series circuit of a commercial power supply and a high-voltage transformer is connected between the connection point of the first and second diodes and the connection point of the first and second semiconductor switch elements that can be brought into reverse conduction, and high-voltage output of the high-voltage transformer supplies power to a magnetron through a high-voltage rectification circuit.
Thus, the first and second semiconductor switch elements are turned on and off complementarily, whereby if the commercial power supply has a positive voltage polarity, the voltage provided by boosting the voltage of the commercial power supply is applied to the second capacitor and if the commercial power supply has an opposite voltage polarity, the voltage provided by boosting the voltage of the commercial power supply is applied to the first capacitor. Since the voltage applied to the primary winding of the high-voltage transformer depends on the boosted voltage, the voltage required for the magnetron to oscillate can always be applied to the primary winding of the high-voltage transformer even in a time period over which the voltage of the commercial power supply is low, and input current can be allowed to flow over almost all regions of the commercial power supply, so that input current with small distortion can be provided. Since the first and second semiconductor switch elements can perform the inverter operation of allowing a high-frequency current to flow into the primary winding of the high-voltage transformer and the operation of applying the boosted voltage to the first and second capacitors at a time, the inverter can be made up of a minimum number of components and the inverter circuit can be miniaturized. In the circuit operation, the first and second diodes are turned off by the semiconductor switch elements and the circuit mode is switched and thus the diodes can be designed with importance attached to the forward on voltage without receiving any restriction on the switching speed, the losses of the diodes can be extremely lessened, and the inverter circuit can be made highly efficient.
In the described configuration, in the invention, each of on-off duty ratios of the first and second semiconductor switch elements is set to 50% in the vicinity of the polarity change point where the polarity changes, and one semiconductor switch element for governing the step-up charging-up function and the inverter function and the other semiconductor switch element for governing only the inverter function are switched in the vicinity of the polarity change point. According to such means, in the vicinity of the polarity change point where the polarity changes, the one semiconductor switch element for governing the step-up charging-up function and the inverter function can be switched upon completion of charging and discharging of charge-up capacitor, so that input current whose harmonic component is cut can be provided at a stable high power factor.
Further, to solve the above-described problems, according to the invention, there is provided a magnetron drive power supply comprising a series connection body of first and second semiconductor switches, first and second fly-wheel diodes in inverse parallel to the first and second semiconductor switches, a series connection body of first and second rectification diodes connected in parallel to the first and second semiconductor switches, first and second capacitors connected in parallel to the first and second rectification diodes, a commercial power supply and the primary winding of a high-voltage transformer connected in series to each other, connected between the connection point of the first and second semiconductor switches and the connection point of the first and second rectification diodes, and a high-voltage rectification circuit and a magnetron connected to output of the secondary winding of the high-voltage transformer, characterized in that the first and second fly-wheel diodes and the first and second rectification diodes are housed in one package.
Thus, the diodes can be used with no waste and moreover the need for containing a diode in the semiconductor switch is eliminated, so that an inexpensive magnetron drive power supply can be provided.